FIG. 6 shows an example of a circuit network measurement device with many measurement ports; in this example, there are 8 ports. This arrangement is generally called an "S parameter test system" or "network analyzer system". These systems consist of a network analyzer 11, shown on top in the figure, and a lower test set 12. However, there are also devices in which both parts are combined into one apparatus. Here, we shall take the general separate-type S parameter test system as an example.
Test set 12 conducts measurement signals from a source in network analyzer 11 to a circuit being measured, separates the transmitted and reflected signals to measure the S parameters, and sends them to a receiver in the network analyzer. Transmission of measurement signals between the network analyzer and the test set is performed through four coaxial cables 13. In addition, although not shown in the figure, suitable cables also connect the network analyzer and the test set at their rear panels for power supply and control signals. Moreover, there are also cases in which the circuit network measurement device is controlled by a computer.
Connectors 14 on test set 12 serve as measurement ports (below, "measurement ports" are referred to as "ports"). The numbers placed on the connectors are the numbers of the ports. In measurements of the circuit network, when the circuit is connected directly to connectors 14 and measured, the connectors are the ports. However, the circuit network to be measured is rarely connected directly to connectors 14; generally, coaxial cables 15 are connected to connectors 14, and the circuit network to be measured is connected to connectors 16 at the front ends of coaxial cables 15. At such time, connectors 16 at the ends of cables 15 become the ports. Furthermore, in FIG. 6, only some of the coaxial cables are shown; the connectors on the test set side of the coaxial cables are omitted from the figure.
Among methods for calibrating this circuit network measurement device, the full two-port calibration method is known as the best method. Full two-port calibration consists of (a) one-port calibration, (b) isolation calibration, (c) through calibration, and (d) load match calibration. In a one-port calibration, three known impedances are prepared as standards; the port is calibrated by successively connecting these standards. The same calibration is also performed on the other ports. Ordinarily, the three known impedances used are open, short, and load.
Isolation calibration is performed in order to correct errors caused by signal leakage between the ports. Through calibration is performed to correct errors in transmission coefficients between the ports. Load match calibration is performed in order to correct errors produced by mis-matching when the ports receive the transmitted signals. The measurement values in these four calibrations are subjected to calculation processing to correct the errors. Details of a full two-port calibration method are given in the following reference: "Accuracy Enhancement Fundamentals--Characterizing Microwave Systematic Errors," HP 8753C Network Analyzer Operating Manual, Reference Section, Appendix to Chapter 5.
Not only the through calibration is a part of the full two-port calibration as mentioned above, but it is also performed independently.
The procedure for performing a through calibration is as follows: a through standard with a known transmission coefficient is connected between the ports and the transmission coefficient is measured; any error is corrected by the difference between the value of the through standard and the measured value. The through standard is generally a coaxial cable or coaxial connector with a negligible loss. However, in cases in which the connectors are connected directly to each other, as in the case in which the ports are APC7 connectors on the ends of the coaxial cables, the through standards are unnecessary.
In the calibration method described above, the only difference between the through calibration as part of the full two-port calibration and the through calibration performed independently is in the equation for correcting the errors. In the explanation given below, the equation for the through calibration performed independently is given, and the problems of prior art calibration methods are explained.
S parameters are used to display the characteristics of a circuit network at high frequencies. FIG. 4 shows a two-port circuit network. The incident wave, from port 1 in the figure to the circuit network, is designated as "a1", the reflected wave as "b1", the incident wave from port 2 to the circuit network as "a2", and the reflected wave as "b2". In this case, the relationship between these is expressed as shown in the following equation, using the S parameters: ##EQU1##
The transmission coefficient of the signal from port 1 to port 2 is S21; it is expressed by the following equation: ##EQU2##
The error model for the transmission coefficient S21 of the circuit network of FIG. 4 is shown by the signal flow diagram in FIG. 5. If the true value of the transmission coefficient of the circuit network is S21a and the error of the measurement system is Et, the measured value S21m becomes EQU S21m=S21a.multidot.Et
The through calibration is performed by connecting the through standard, which has the known value S21as for the transmission coefficient, between the ports, and measuring S21. If this measured value is S21ms, the error Et of the measurement system is expressed by the following equation: EQU Et=S21ms/S21as
Thus, if Et is obtained beforehand by through calibration, the value S21a in which the error of the measurement system is corrected can be obtained for any measurement. That is, the equation to correct the error becomes the equation below: EQU S21a=S21m/Et (A)
In the case in which the through calibration is performed for S12, for the opposite transmission direction from that of S21, the connections are switched: port 1 in the S21 measurement is connected to the source side of network analyzer 11, and port 2 is connected to the receiver side; port 2 in the S12 measurement is connected to the source side, and port 1 is connected to the receiver side. This change is performed in test set 12 by the logical control of the network analyzer. Further, there is no need to change the connections of the through standard.
Here, the case is considered in which through calibration is performed for all transmission directions in a circuit network measurement device, with n as the number of ports. The connection of the through standard must be performed nC.sub.2 =n(n-1)/2 times, the number of combinations when two are selected from n. Moreover, since the measurements are performed for both the regular and reverse directions, n(n-1) measurements are performed.
Therefore, the problem with the prior art is that, as n becomes larger, the number of connections of the standard and the measurement time increase as the square, and the burden of the calibration measurement also becomes large.